Multilayer ceramic capacitor

ABSTRACT

In a multilayer ceramic capacitor including first and second inner electrodes alternately arranged with respect to a stacking direction of a multilayer body, a polarity based on a direction of voltage applied between a first outer electrode and a second outer electrode is determined such that the first inner electrodes function as positive electrodes and the second inner electrodes function as negative electrodes. The first inner electrodes have a first metal composition including Cu as a main component, and the second inner electrodes have a second metal composition including Cu as a main component, and at least one metal element selected from Au, Pt, Ir, Pd, Os, Ag, Rh, and Ru, which have standard electrode potentials higher than that of Cu, as an additive component.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to Japanese Patent Application No. 2021-058922 filed on Mar. 31, 2021 and is a Continuation Application of PCT Application No. PCT/JP2022/015243 filed on Mar. 29, 2022. The entire contents of each application are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a multilayer ceramic capacitor, in particular, to a metal composition of an inner electrode of a multilayer ceramic capacitor.

2. Description of the Related Art

Recent development in electronic technology requires multilayer ceramic capacitors with smaller size and higher capacitance. In order to meet these requirements, dielectric layers of multilayer ceramic capacitors are becoming thinner. However, thickness reduction of dielectric layers relatively increases the intensity of electric field applied to each layer. Therefore, it is desirable to improve reliability during voltage application.

In general, a multilayer ceramic capacitor includes a multilayer body that includes multiple dielectric layers that are stacked on top of each other and multiple inner electrodes disposed along interfaces between the dielectric layers, and multiple outer electrodes that are formed on outer surfaces of the multilayer body and are electrically connected to the inner electrodes. Here, there is a known inner electrode that includes Ni as a main component, as described in Japanese Unexamined Patent Application Publication No. 11-283867.

SUMMARY OF THE INVENTION

However, the issue with the inner electrode including Ni as a main component is that the reliability during voltage application is insufficient to meet the recent demand for miniaturization and higher capacitance.

Preferred embodiments of the present invention provide multilayer ceramic capacitors that each have thinner dielectric layers and exhibit excellent reliability even when voltage of a high electric field intensity is applied.

A multilayer ceramic capacitor according to a preferred embodiment of the present invention includes a multilayer body that includes multiple dielectric layers that are made of a ceramic and are stacked on top of each other, and multiple inner electrodes along interfaces between the dielectric layers, and multiple outer electrodes on outer surfaces of the multilayer body and electrically connected to the inner electrodes.

The inner electrodes include multiple first inner electrodes and multiple second inner electrodes that are alternately arranged with respect to a stacking direction of the multilayer body, and the outer electrodes include a first outer electrode electrically connected to the first inner electrodes and a second outer electrode electrically connected to the second inner electrodes.

In a preferred embodiment of the present invention, a polarity based on a direction of voltage applied between the first outer electrode and the second outer electrode is determined such that the first inner electrodes function as positive electrodes and the second inner electrodes function as negative electrodes, the first inner electrodes have a first metal composition including Cu as a main component, and the second inner electrodes have a second metal composition including Cu as a main component and at least one metal element selected from Au, Pt, Ir, Pd, Os, Ag, Rh, and Ru, which have standard electrode potentials higher than that of Cu, as an additive component.

Note that the first metal composition and the second metal composition are different from each other in terms of at least one of the types and contents of the constituent elements.

Regarding the metal compositions of the inner electrodes, the “main component” refers to a metal element having the highest content among all metal elements, more specifically, a content higher than or equal to 50%.

According to preferred embodiments of the present invention, insulation degradation of a multilayer ceramic capacitor during voltage application is reduced or prevented, and thus multilayer ceramic capacitors each with excellent reliability can be obtained.

In addition, multilayer ceramic capacitors with good high frequency characteristics can be obtained.

The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically showing a multilayer ceramic capacitor 1 according to a preferred embodiment of the present invention.

FIG. 2 is a diagram showing standard electrode potentials of metal elements included in first inner electrodes (positive electrodes) 4 and second inner electrodes (negative electrodes) 5 in the multilayer ceramic capacitor 1 illustrated in FIG. 1 .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The structure of a multilayer ceramic capacitor 1 according to a preferred embodiment of the present invention will now be described with reference to FIG. 1 .

The multilayer ceramic capacitor 1 includes a multilayer body 2. The multilayer body 2 includes multiple dielectric layers 3 that are made of a ceramic and are stacked on top of each other, and multiple inner electrodes 4 and 5 that are disposed along interfaces between the dielectric layers 3. The inner electrodes 4 and 5 are classified into multiple first inner electrodes 4 and multiple second inner electrodes 5 that are alternately arranged with respect to the stacking direction of the multilayer body 3. An outer electrode 6 and an outer electrode 7 are respectively disposed on outer surfaces, more specifically, opposing end surfaces, of the multilayer body 2. The outer electrodes 6 and 7 are classified into a first outer electrode 6 electrically connected to the first inner electrodes 4, and a second outer electrode 7 electrically connected to the second inner electrodes 5.

The compositions of the inner electrodes 4 and 5 are described below. The outer electrodes 6 and 7 include, for example, Ag or Cu as a main component of the conductive component. The dielectric layers 3 preferably include a dielectric ceramic that includes, as a main component, a perovskite compound that includes Ba and Ti (however, some of Ba may be substituted with Ca, and some of Ti may be substituted with Zr). In particular, high permittivity is exhibited when the main component of dielectric layers 3 is BaTiO₃, and the multilayer ceramic capacitor 1 exhibits excellent reliability. Note that the dielectric layers 3 may include, in addition to the aforementioned main component, rare earth elements, Mn, Mg, Si, etc., for example, as sub components.

The raw material powder of the dielectric ceramic is, for example, produced by a solid phase synthesis method. Specifically, first, compound powders of oxides, carbonates, etc., that include constituent elements of the main component are mixed in predetermined proportions and calcined. Instead of the solid phase synthesis method, a hydrothermal method and other appropriate methods may be applied. Here, the dielectric ceramic may include alkali metals, transition metals, Cl, S, P, Hf, etc., as long as the amounts thereof do not obstruct the advantageous effects of preferred embodiments of the present invention.

The multilayer ceramic capacitor 1 is, for example, produced as follows. A ceramic slurry is prepared by using raw material powder of the dielectric ceramic obtained as described above. Next, ceramic green sheets are formed by a sheet forming method or the like. Next, a conductive paste that will form the inner electrodes 4 and 5 are applied, by printing or the like, to particular ceramic green sheets selected from among these ceramic green sheets. Then the ceramic green sheets are stacked on top of each other and pressure-bonded to obtain a green multilayer body. Next, the green multilayer body is fired. In this firing step, dielectric layers 3 including a dielectric ceramic are obtained. Subsequently, outer electrodes 6 and 7 are formed on end surfaces of the multilayer body 3 by baking or the like.

One of the unique features of the multilayer ceramic capacitor 1 is that the polarity based on the direction of the voltage applied between the first outer electrode 6 and the second outer electrode 7 is determined such that the first inner electrodes 4 function as positive electrodes and the second inner electrodes 5 function as negative electrodes in actual use. Therefore, although not illustrated, a mark that indicates the polarity is preferably put on, for example, an outer surface of the multilayer ceramic capacitor 1.

In this regard, a multilayer ceramic capacitor according to a preferred embodiment of the present invention is not limited to the two-terminal type including the first outer electrode 6 and the second outer electrode 7 as shown in FIG. 1 , but may also be a multi-terminal type including three or more outer electrodes. In this case, the voltage may be applied between particular two sets of outer electrodes selected from among the three or more outer electrodes, in other words, between at least one first outer electrode and at least one second outer electrode, such that the first inner electrodes function as positive electrodes and the second inner electrodes function as negative electrodes.

Another of the unique features of the multilayer ceramic capacitor 1 is that the metal compositions of the first inner electrodes 4 and the second inner electrodes 5 are selected as follows. That is, the first inner electrodes 4 functioning as positive electrodes have a first metal composition including Cu as a main component, and the second inner electrodes 5 functioning as negative electrodes have a second metal composition including Cu as a main component, and at least one metal element selected from Au, Pt, Ir, Pd, Os, Ag, Rh, and Ru, which have standard electrode potentials higher than that of Cu, as an additive component. Au, Pt, Ir, Pd, Os, Ag, Rh, and Ru included in the second metal composition are noble metals.

Such a choice of the metal compositions of the first inner electrodes 4 and the second inner electrodes 5 is based on the following findings.

Although the insulation degradation mechanism of typical multilayer ceramic capacitors is not yet elucidated, it is known that negative electrode segregation of oxygen ions (positive electrode segregation of oxygen vacancies) caused by voltage application triggers the degradation. Therefore, it is expected that the insulation degradation of the multilayer ceramic capacitor can be reduced or prevented by reducing or preventing the negative electrode segregation of oxygen ions. In this regard, a possible approach is to have positive electrodes include an element that gives a stable oxide and to have negative electrodes include an element that gives unstable oxide. According to this idea, negative electrode segregation can be reduced or prevented by allowing reduction reactions (release of oxygen ions) to occur in the negative electrodes.

More specifically, regarding the metal compositions of the first inner electrodes 4 and the second inner electrodes 5, metal elements that have a tendency to undergo an increase in valency (low standard electrode potential) are used in the first inner electrodes 4 that function as positive electrodes, and metal elements that have a tendency to undergo a decrease in valency (high standard electrode potential) are used in the second inner electrodes 5 that function as negative electrodes. A standard electrode potential is a value inherent to the element, the lower the value, the more stable the oxide, and the higher the value, the more unstable the oxide.

That is, as illustrated in FIG. 2 , the second inner electrodes 5 that function as negative electrodes are to include metal elements having standard electrode potentials higher than the standard electrode potentials of the metal elements in the first inner electrodes 4 that function as positive electrodes. In FIG. 2 , the standard electrode potential of the first metal composition of the first inner electrodes 4 functioning as positive electrodes is to be within the range A, and the standard electrode potential of the second metal composition of the second inner electrodes 5 functioning as negative electrodes is to be within the range B.

The standard electrode potentials of the metal elements that can be included in the metal compositions of the inner electrodes 4 and 5 are, in the ascending order:

-   -   +0.34 V for Cu,     -   +0.46 V for Ru,     -   +0.76 V for Rh,     -   +0.8 V for Ag,     -   +0.9 V for Os,     -   +0.92 V for Pd,     -   +1.16 V for Ir,     -   +1.19 V for Pt, and     -   +1.52 V for Au.

Whereas the standard electrode potential of Cu included as a main component of the first metal composition of the first inner electrodes 4 is +0.34 V, Au, Pt, Ir, Pd, Os, Ag, Rh, and Ru added to the second metal composition of the second inner electrodes 5 all have a standard electrode potential higher than that of Cu.

Thus, at least one metal element selected from Au, Pt, Ir, Pd, Os, Ag, Rh, and Ru, which are added to the second metal composition of the second inner electrodes 5 functioning as negative electrodes and have higher standard electrode potentials than Cu, can induce the reduction reaction (release of oxygen ions) in the second inner electrodes 5 functioning as negative electrodes, and thus can reduced or prevent negative electrode segregation of oxygen ions.

As mentioned above, the higher the standard electrode potential, the more unstable the oxide. Thus, the effect of reducing or preventing the negative electrode segregation of oxygen ions brought about by the metal elements that can be included in the second metal composition of the second inner electrodes 5 described above increases in the order of Ru, Rh, Ag, Os, Pd, Ir, Pt, and Au, which is the ascending order of the standard electrode potential.

Thus, according to this preferred embodiment focusing on the redox reaction in the inner electrodes 4 and 5, the negative electrode segregation of oxygen ions (positive electrode segregation of oxygen vacancies) caused by voltage application is reduced or prevented by utilizing the redox reaction. As a result, the insulation degradation of the multilayer ceramic capacitor 1 during voltage application can be reduced or prevented, and a multilayer ceramic capacitor 1 having excellent reliability can be obtained.

The first metal composition of the first inner electrodes 4 is preferably free of metal elements having standard electrode potentials higher than that of Cu. This is because the first metal composition can advantageously retain the relatively low standard electrode potential.

In addition, the first metal composition of the first inner electrodes 4 and the second metal composition of the second inner electrodes 5 both include Cu as the main component, the high frequency characteristics of the multilayer ceramic capacitor 1 can be improved.

It should be noted that multiple metal elements of the second metal composition of the second inner electrodes 5 are included in the conductive paste applied to the ceramic green sheets used to produce the multilayer ceramic capacitor 1. Here, the multiple metal elements may be preliminarily prepared as an alloy or intermetallic compound including these metal elements and then included in the conductive paste, or may be separately included in the conductive paste. Also, when multiple metal elements are included in the first metal composition of the first inner electrodes 4, these metal elements may be preliminarily prepared as an alloy or intermetallic compound including these metal elements and then included in the conductive paste, or may be separately included in the conductive paste.

These multiple metal elements included in the second metal composition of the second inner electrodes 5 may be alloyed in the multilayer ceramic capacitor 1 as a product.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims. 

What is claimed is:
 1. A multilayer ceramic capacitor comprising: a multilayer body including a plurality of dielectric layers that are made of a ceramic and are stacked on top of each other, and a plurality of inner electrodes along interfaces between the plurality of dielectric layers; and a plurality of outer electrodes on outer surfaces of the multilayer body and electrically connected to the plurality of inner electrodes; wherein the plurality of inner electrodes include a plurality of first inner electrodes and a plurality of second inner electrodes that are alternately arranged with respect to a stacking direction of the multilayer body; the plurality of outer electrodes include a first outer electrode electrically connected to the plurality of first inner electrodes and a second outer electrode electrically connected to the plurality of second inner electrodes; a polarity based on a direction of voltage applied between the first outer electrode and the second outer electrode is determined such that the plurality of first inner electrodes function as positive electrodes and the plurality of second inner electrodes function as negative electrodes; the plurality of first inner electrodes have a first metal composition that includes Cu as a main component; and the plurality of second inner electrodes have a second metal composition that includes Cu as a main component and at least one metal element selected from Au, Pt, Ir, Pd, Os, Ag, Rh, and Ru, which have a standard electrode potential higher than that of Cu, as an additive component.
 2. The multilayer ceramic capacitor according to claim 1, wherein the first metal composition of the plurality of first inner electrodes does not include a metal element having a standard electrode potential higher than that of Cu.
 3. The multilayer ceramic capacitor according to claim 1, wherein the first outer electrode and the second outer electrode include Ag or Cu.
 4. The multilayer ceramic capacitor according to claim 1, wherein the plurality of dielectric layers include as a main component a perovskite compound including Ba and Ti.
 5. The multilayer ceramic capacitor according to claim 4, wherein some of Ba is substituted with Ca.
 6. The multilayer ceramic capacitor according to claim 4, wherein some of Ti is substituted with Zr.
 7. The multilayer ceramic capacitor according to claim 1, wherein the plurality of dielectric layers include BaTiO₃.
 8. The multilayer ceramic capacitor according to claim 1, wherein the plurality of dielectric layers include a dielectric ceramic and at least one of rare earth elements, Mn, Mg, or Si.
 9. The multilayer ceramic capacitor according to claim 1, wherein the plurality of dielectric layers include at least one of alkali metals, transition metals, Cl, S, P or Hf.
 10. The multilayer ceramic capacitor according to claim 1, wherein the multilayer ceramic capacitor is a two-terminal capacitor.
 11. The multilayer ceramic capacitor according to claim 1, wherein the multilayer ceramic capacitor is a multi-terminal capacitor including three or more outer electrodes.
 12. The multilayer ceramic capacitor according to claim 1, wherein the second composition of the plurality of second inner electrodes includes multiple metal elements.
 13. The multilayer ceramic capacitor according to claim 12, wherein the multiple metal elements in the second composition are alloyed as a product.
 14. The multilayer ceramic capacitor according to claim 1, wherein the first composition of the plurality of first inner electrodes includes multiple metal elements. 